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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Exceptions</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="C++, exception, error, exception neutrality, exception safety, exception propagation, -fno-exceptions" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="using.html" title="Chapter 3. Using" /><link rel="prev" href="using_concurrency.html" title="Concurrency" /><link rel="next" href="debug.html" title="Debugging Support" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Exceptions</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="using_concurrency.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td width="20%" align="right"> <a accesskey="n" href="debug.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.intro.using.exceptions"></a>Exceptions</h2></div></div></div><p>
The C++ language provides language support for stack unwinding
with <code class="literal">try</code> and <code class="literal">catch</code> blocks and
the <code class="literal">throw</code> keyword.
</p><p>
These are very powerful constructs, and require some thought when
applied to the standard library in order to yield components that work
efficiently while cleaning up resources when unexpectedly killed via
exceptional circumstances.
</p><p>
Two general topics of discussion follow:
exception neutrality and exception safety.
</p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.safety"></a>Exception Safety</h3></div></div></div><p>
    What is exception-safe code?
  </p><p>
    Will define this as reasonable and well-defined behavior by classes
    and functions from the standard library when used by user-defined
    classes and functions that are themselves exception safe.
  </p><p>
    Please note that using exceptions in combination with templates
    imposes an additional requirement for exception
    safety. Instantiating types are required to have destructors that
    do no throw.
  </p><p>
    Using the layered approach from Abrahams, can classify library
    components as providing set levels of safety. These will be called
    exception guarantees, and can be divided into three categories.
  </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
    One. Don't throw.
  </p><p>
    As specified in 23.2.1 general container requirements. Applicable
    to container and string classes.
  </p><p>
    Member
    functions <code class="function">erase</code>, <code class="function">pop_back</code>, <code class="function">pop_front</code>, <code class="function">swap</code>, <code class="function">clear</code>. And <span class="type">iterator</span>
    copy constructor and assignment operator.
  </p></li><li class="listitem"><p>
    Two. Don't leak resources when exceptions are thrown. This is
    also referred to as the <span class="quote">“<span class="quote">basic</span>”</span> exception safety guarantee.
  </p><p>
    This applicable throughout the standard library.
  </p></li><li class="listitem"><p>
    Three. Commit-or-rollback semantics.  This is
    referred to as <span class="quote">“<span class="quote">strong</span>”</span> exception safety guarantee.
  </p><p>
    As specified in 23.2.1 general container requirements. Applicable
    to container and string classes.
  </p><p>
    Member functions <code class="function">insert</code> of a single
    element, <code class="function">push_back</code>, <code class="function">push_front</code>,
    and <code class="function">rehash</code>.
  </p></li></ul></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.propagating"></a>Exception Neutrality</h3></div></div></div><p>
    Simply put, once thrown an exception object should continue in
    flight unless handled explicitly. In practice, this means
    propagating exceptions should not be swallowed in
    gratuitous <code class="literal">catch(...)</code> blocks. Instead,
    matching <code class="literal">try</code> and <code class="literal">catch</code>
    blocks should have specific catch handlers and allow un-handed
    exception objects to propagate. If a
    terminating <code class="literal">catch(...)</code> blocks exist then it
    should end with a <code class="literal">throw</code> to re-throw the current
    exception.
  </p><p>
    Why do this?
  </p><p>
    By allowing exception objects to propagate, a more flexible
    approach to error handling is made possible (although not
    required.) Instead of dealing with an error immediately, one can
    allow the exception to propagate up until sufficient context is
    available and the choice of exiting or retrying can be made in an
    informed manner.
  </p><p>
    Unfortunately, this tends to be more of a guideline than a strict
    rule as applied to the standard library. As such, the following is
    a list of known problem areas where exceptions are not propagated.
  </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
      Input/Output
    </p><p>
    The destructor <code class="function">ios_base::Init::~Init()</code>
    swallows all exceptions from <code class="function">flush</code> called on
    all open streams at termination.
  </p><p>
    All formatted input in <code class="classname">basic_istream</code> or
    formatted output in <code class="classname">basic_ostream</code> can be
    configured to swallow exceptions
    when <code class="function">exceptions</code> is set to
    ignore <span class="type">ios_base::badbit</span>.
  </p><p>
    Functions that have been registered
    with <code class="function">ios_base::register_callback</code> swallow all
    exceptions when called as part of a callback event.
  </p><p>
    When closing the underlying
    file, <code class="function">basic_filebuf::close</code> will swallow
    (non-cancellation) exceptions thrown and return <code class="literal">NULL</code>.
  </p></li><li class="listitem"><p>
      Thread
    </p><p>
      The constructors of <code class="classname">thread</code> that take a
      callable function argument swallow all exceptions resulting from
      executing the function argument.
    </p></li></ul></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.alloc"></a>Memory allocation</h3></div></div></div><p>
    When the program throws an exception the runtime will obtain storage for
    a <code class="code">__cxa_exception</code> header and the thrown object itself.
    Libstdc++ will try to use <code class="code">malloc</code> to obtain storage,
    but provides an emergency buffer to be used if malloc fails,
    as described by the <a class="link" href="https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html#imp-emergency" target="_top">Itanium
    exception handling ABI</a>.
  </p><p>
    Contrary to the ABI, the libstdc++ emergency buffer is not always 64kB,
    and does not always allocate 1kB chunks. The buffer is used as a pool for
    variable-sized allocations, so that it doesn't waste space for smaller
    exception objects such as <code class="code">std::bad_alloc</code>.
    The total size of the buffer is scaled appropriately for the target.
    Specifically it depends on <code class="code">sizeof(void*)</code>, so that a 64-bit
    system uses a larger pool than a 32-bit system. This is done because for
    32-bit systems the exception objects (and the exception header) require
    less space, and core counts and thread counts are typically lower as well.
  </p><p>
    By default, libstdc++ will use <code class="code">malloc</code> to allocate the buffer
    on program startup.
    <a class="xref" href="configure.html" title="Configure">Configuring</a> libstdc++ with the
    <code class="code">--enable-libstdcxx-static-eh-pool</code> option will make it
    use a static buffer instead of using <code class="code">malloc</code>.
  </p><p>
    The buffer size is chosen automatically, but can be overridden
    by configuring with <code class="code">--with-libstdcxx-eh-pool-obj-count=NUM</code>,
    where <code class="code">NUM</code> is the number of simultaneous allocations that
    should be supported. The size of the pool will be sufficient for
    <code class="code">NUM</code> exceptions of <code class="code">6 * sizeof(void*)</code> bytes,
    plus another <code class="code">NUM</code> exceptions captured in
    <code class="classname">std::exception_ptr</code> and rethrown using
    <code class="code">std::rethrow_exception</code>. The buffer size determined by the
    obj-count value applies whether the buffer is reserved as static storage
    or is allocated dynamically.
    Setting obj-count to zero will disable the pool, so that no emergency
    buffer is present.
  </p><p>
    For a dynamic buffer, the default size can also be changed at runtime,
    per-process, via the <code class="literal">GLIBCXX_TUNABLES</code> environment
    variable.
    The <code class="literal">GLIBCXX_TUNABLES</code> environment variable should be
    a string of colon-separated <span class="emphasis"><em>name=value</em></span> pairs. The
    following names will be recognized, with the specified semantics:
  </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="code">glibcxx.eh_pool.obj_count</code></span></dt><dd>
    The number of exception objects to provide space for in the pool.
    The value must be a non-negative integer and has the same meaning as the
    <code class="code">--with-libstdcxx-eh-pool-obj-count</code> option for
    <code class="filename">configure</code>.
  </dd><dt><span class="term"><code class="code">glibcxx.eh_pool.obj_size</code></span></dt><dd>
    The expected size of exception objects that the pool might get used for.
    The value must be a positive integer, and is measured in units of
    <code class="code">sizeof(void*)</code>. The default value is <code class="literal">6</code>
    which is large enough to store any exception type thrown by libstdc++.
    Exceptions larger than this can still be allocated from the pool,
    but larger exceptions will exhaust the pool more rapidly.
  </dd></dl></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.no"></a>Doing without</h3></div></div></div><p>
    C++ is a language that strives to be as efficient as is possible
    in delivering features. As such, considerable care is used by both
    language implementer and designers to make sure unused features
    not impose hidden or unexpected costs. The GNU system tries to be
    as flexible and as configurable as possible. So, it should come as
    no surprise that GNU C++ provides an optional language extension,
    spelled <code class="literal">-fno-exceptions</code>, as a way to excise the
    implicitly generated magic necessary to
    support <code class="literal">try</code> and <code class="literal">catch</code> blocks
    and thrown objects. (Language support
    for <code class="literal">-fno-exceptions</code> is documented in the GNU
    GCC <a class="link" href="http://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code-Gen-Options" target="_top">manual</a>.)
  </p><p>Before detailing the library support
    for <code class="literal">-fno-exceptions</code>, first a passing note on
    the things lost when this flag is used: it will break exceptions
    trying to pass through code compiled
    with <code class="literal">-fno-exceptions</code> whether or not that code
    has any <code class="literal">try</code> or <code class="literal">catch</code>
    constructs. If you might have some code that throws, you shouldn't
    use <code class="literal">-fno-exceptions</code>. If you have some code that
    uses <code class="literal">try</code> or <code class="literal">catch</code>, you
    shouldn't use <code class="literal">-fno-exceptions</code>.
  </p><p>
    And what it to be gained, tinkering in the back alleys with a
    language like this? Exception handling overhead can be measured
    in the size of the executable binary, and varies with the
    capabilities of the underlying operating system and specific
    configuration of the C++ compiler. On recent hardware with GNU
    system software of the same age, the combined code and data size
    overhead for enabling exception handling is around 7%. Of course,
    if code size is of singular concern than using the appropriate
    optimizer setting with exception handling enabled
    (ie, <code class="literal">-Os -fexceptions</code>) may save up to twice
    that, and preserve error checking.
  </p><p>
    So. Hell bent, we race down the slippery track, knowing the brakes
    are a little soft and that the right front wheel has a tendency to
    wobble at speed. Go on: detail the standard library support
    for <code class="literal">-fno-exceptions</code>.
  </p><p>
    In sum, valid C++ code with exception handling is transformed into
    a dialect without exception handling. In detailed steps: all use
    of the C++
    keywords <code class="literal">try</code>, <code class="literal">catch</code>,
    and <code class="literal">throw</code> in the standard library have been
    permanently replaced with the pre-processor controlled equivalents
    spelled <code class="literal">__try</code>, <code class="literal">__catch</code>,
    and <code class="literal">__throw_exception_again</code>. They are defined
    as follows.
  </p><pre class="programlisting">
#if __cpp_exceptions
# define __try      try
# define __catch(X) catch(X)
# define __throw_exception_again throw
#else
# define __try      if (true)
# define __catch(X) if (false)
# define __throw_exception_again
#endif
</pre><p>
  In addition, for every object derived from
  class <code class="classname">exception</code>, there exists a corresponding
  function with C language linkage. An example:
</p><pre class="programlisting">
#if __cpp_exceptions
  void __throw_bad_exception(void)
  { throw bad_exception(); }
#else
  void __throw_bad_exception(void)
  { abort(); }
#endif
</pre><p>
  The last language feature needing to be transformed
  by <code class="literal">-fno-exceptions</code> is treatment of exception
  specifications on member functions. Fortunately, the compiler deals
  with this by ignoring exception specifications and so no alternate
  source markup is needed.
</p><p>
  By using this combination of language re-specification by the
  compiler, and the pre-processor tricks and the functional
  indirection layer for thrown exception objects by the library,
  libstdc++ files can be compiled
  with <code class="literal">-fno-exceptions</code>.
</p><p>
 User code that uses C++ keywords
 like <code class="literal">throw</code>, <code class="literal">try</code>,
 and <code class="literal">catch</code> will produce errors even if the user
 code has included libstdc++ headers and is using constructs
 like <code class="classname">basic_iostream</code>. Even though the standard
 library has been transformed, user code may need modification. User
  code that attempts or expects to do error checking on standard
  library components compiled with exception handling disabled should
  be evaluated and potentially made conditional.
</p><p>
  Some issues remain with this approach (see bugzilla entry
  25191). Code paths are not equivalent, in
  particular <code class="literal">catch</code> blocks are not evaluated. Also
  problematic are <code class="literal">throw</code> expressions expecting a
  user-defined throw handler. Known problem areas in the standard
  library include using an instance
  of <code class="classname">basic_istream</code>
  with <code class="function">exceptions</code> set to specific
  <span class="type">ios_base::iostate</span> conditions, or
  cascading <code class="literal">catch</code> blocks that dispatch error
  handling or recovery efforts based on the type of exception object
  thrown.
</p><p>
  Oh, and by the way: none of this hackery is at all
  special. (Although perhaps well-deserving of a raised eyebrow.)
  Support continues to evolve and may change in the future. Similar
  and even additional techniques are used in other C++ libraries and
  compilers.
</p><p>
 C++ hackers with a bent for language and control-flow purity have
  been successfully consoled by grizzled C veterans lamenting the
  substitution of the C language keyword
  <code class="literal">const</code> with the uglified
  doppelganger <code class="literal">__const</code>.
</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.compat"></a>Compatibility</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="using.exception.compat.c"></a>With <code class="literal">C</code></h4></div></div></div><p>
  C language code that is expecting to interoperate with C++ should be
  compiled with <code class="literal">-fexceptions</code>. This will make
  debugging a C language function called as part of C++-induced stack
  unwinding possible.
</p><p>
  In particular, unwinding into a frame with no exception handling
data will cause a runtime abort. If the unwinder runs out of unwind
info before it finds a handler, <code class="function">std::terminate()</code>
is called.
</p><p>
  Please note that most development environments should take care of
  getting these details right. For GNU systems, all appropriate parts
  of the GNU C library are already compiled
  with <code class="literal">-fexceptions</code>.
</p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="using.exception.compat.posix"></a>With <code class="literal">POSIX</code> thread cancellation</h4></div></div></div><p>
  GNU systems re-use some of the exception handling mechanisms to
  track control flow for <code class="literal">POSIX</code> thread cancellation.
</p><p>
  Cancellation points are functions defined by POSIX as worthy of
  special treatment. The standard library may use some of these
  functions to implement parts of the ISO C++ standard or depend on
  them for extensions.
</p><p>
  Of note:
</p><p>
  <code class="function">nanosleep</code>,
  <code class="function">read</code>, <code class="function">write</code>, <code class="function">open</code>, <code class="function">close</code>,
  and <code class="function">wait</code>.
</p><p>
  The parts of libstdc++ that use C library functions marked as
  cancellation points should take pains to be exception neutral.
  Failing this, <code class="literal">catch</code> blocks have been augmented to
  show that the POSIX cancellation object is in flight.
</p><p>
  This augmentation adds a <code class="literal">catch</code> block
  for <code class="classname">__cxxabiv1::__forced_unwind</code>, which is the
  object representing the POSIX cancellation object. Like so:
</p><pre class="programlisting">
  catch(const __cxxabiv1::__forced_unwind&amp;)
  {
    this-&gt;_M_setstate(ios_base::badbit);
    throw;
  }
  catch(...)
  { this-&gt;_M_setstate(ios_base::badbit); }
</pre></div></div><div class="bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="using.exceptions.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.2"></a><p><span class="title"><em>
	<a class="link" href="http://www.opengroup.org/austin/" target="_top">
	System Interface Definitions, Issue 7 (IEEE Std. 1003.1-2008)
	</a>
      </em>. </span><span class="pagenums">
      2.9.5 Thread Cancellation
    . </span><span class="copyright">Copyright © 2008 
	The Open Group/The Institute of Electrical and Electronics
	Engineers, Inc.
      . </span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.3"></a><p><span class="title"><em>
	<a class="link" href="https://www.boost.org/community/error_handling.html" target="_top">
	Error and Exception Handling
	</a>
      </em>. </span><span class="author"><span class="firstname">David</span> <span class="surname">Abrahams </span>. </span><span class="publisher"><span class="publishername">
	Boost
      . </span></span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.4"></a><p><span class="title"><em>
	<a class="link" href="https://www.boost.org/community/exception_safety.html" target="_top">
	Exception-Safety in Generic Components
	</a>
      </em>. </span><span class="author"><span class="firstname">David</span> <span class="surname">Abrahams</span>. </span><span class="publisher"><span class="publishername">
	Boost
      . </span></span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.5"></a><p><span class="title"><em>
	<a class="link" href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/1997/N1077.pdf" target="_top">
	Standard Library Exception Policy
	</a>
      </em>. </span><span class="author"><span class="firstname">Matt</span> <span class="surname">Austern</span>. </span><span class="publisher"><span class="publishername">
	WG21 N1077
      . </span></span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.6"></a><p><span class="title"><em>
	<a class="link" href="http://gcc.gnu.org/ml/gcc-patches/2001-03/msg00661.html" target="_top">
	ia64 c++ abi exception handling
	</a>
      </em>. </span><span class="author"><span class="firstname">Richard</span> <span class="surname">Henderson</span>. </span><span class="publisher"><span class="publishername">
	GNU
      . </span></span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.7"></a><p><span class="title"><em>
	<a class="link" href="https://www.stroustrup.com/3rd_safe.pdf" target="_top">
	Appendix E: Standard-Library Exception Safety
	</a>
      </em>. </span><span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.8"></a><p><span class="citetitle"><em class="citetitle">
      Exceptional C++
    </em>. </span><span class="pagenums">
      Exception-Safety Issues and Techniques
    . </span><span class="author"><span class="firstname">Herb</span> <span class="surname">Sutter</span>. </span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.9"></a><p><span class="title"><em>
	<a class="link" href="http://gcc.gnu.org/PR25191" target="_top">
      GCC Bug 25191: exception_defines.h #defines try/catch
	</a>
      </em>. </span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.10"></a><p><span class="title"><em>
	<a class="link" href="https://www.gnu.org/software/libc/manual/html_node/Tunables.html" target="_top">
      Tunables, The GNU C Library
	</a>
      </em>. </span></p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="using_concurrency.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="using.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="debug.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Concurrency </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Debugging Support</td></tr></table></div></body></html>